In recent years, with popularization of smart phones, or the like, data communication traffic volume in mobile service has increased notably. Further, 70% of all data communication traffic is generated and takes place in the indoor communication such as a user home, small business office, and a commercial facility, or the like. In order to accommodate such increase in the traffic demand, the development of base station, which is ultra compact and operable for indoor installation, is ongoing. The transmission power of such ultra compact base station is small compared with that of a base station (hereinafter, referred to as macro base station) arranged outdoor, and the communication area (i.e., cell) of the ultra compact base station is extremely small compared with that of the macro base station. Accordingly, the communication area of the ultra compact base station is referred to as femtocell, and the base station that is ultra compact is referred to as femtocell base station. The femtocell base station has received attention, not only for its ability to accommodate the above stated high demand traffic, but also as an expansion means of coverage (communication area satisfying certain quality) due to its ability to be arranged at locations radio wave struggle to reach such as high rise buildings or underground facilities.
There is an ongoing examination to use the femtocell base station with wireless communication standard of mobile telephone such as 3GPP (3rd Generation Partnership Project) W-CDMA and E-UTRA, or the like, as well as wireless communication standard of wireless MAN (Wireless Metropolitan Area Network) such as IEEE 802.16m, or the like. In W-CDMA (Wideband Code Division Multiple Access) standard, the femtocell base station is referred to as Home NodeB (HNB). Further, in 3GPP E-UTRA (Evolved Universal Terrestrial Radio Access) standard which is referred to as LTE (Long Term Evolution), the femtocell base station is referred to as Home eNodeB (HeNB). The femtocell base station is connected to an upper network (e.g., core network of telecommunication carrier) directly or via femto GW (gateway). The femto GW is referred to as Home NodeB Gateway in W-CDMA standard, and Home eNodeB Gateway in E-UTRA standard.
Note that in this specification, a mobile station which is connected to and communicates with the femtocell base station is referred to as “femtocell mobile station,” and a mobile station which is connected to and communicates with the macro base station is referred to as “macro mobile station.”
When the femtocell base station is used in the W-CDMA system, a data transmission/reception through a dedicated channel accompanying a transmission power control for an uplink and a downlink, and/or a data transmission/reception through a shared channel for a downlink are carried out. Further, when the femtocell base station is used in the E-UTRA system, radio frequency band and time are segmented into resource blocks (PRB; Physical Resource Block). A scheduler arranged at the base station performs PRB allocation, and then a data transmission/reception using the allocated PRBs is carried out. Further, when the femtocell base station is used in the IEEE 802.16m system, OFDMA (Orthogonal Frequency Division Multiple Access) is used, and a radio frequency band is divided into subcarriers. A scheduler arranged at the base station allocates the subcarriers, and a data transmission/reception using the allocated subcarriers is carried out. A bundle of subcarriers corresponds to a resource block in E-UTRA.
Further, as one of the operation configurations for a femtocell, the operation configuration in which only mobile stations which are registered in advance are operable to use the femtocell is known. In 3GPP, a group of mobile stations which are registered in advance is referred to as a Closed Subscriber Group (CSG), and a cell which only mobile stations included in the CSG are permitted to belong is referred to as a CSG cell. In CSG, when a mobile station is within the femtocell and is registered with the femtocell base station, the mobile station is operable to connect to the network via the femtocell base station. On the other hand, the mobile station which is not registered with the femtocell base station is not operable to connect to the network via the femtocell base station, or is limited when communicating with the femtocell base station compared with the mobile station which is registered.
One of the interference scenarios anticipated when using the femto cell base station is that there is a scenario in which the femtocell causes uplink interference to the macro cell. Here, the uplink interference is an interference caused to an uplink reception of the base station (in other words, uplink transmission of mobile station) from uplink transmission of mobile stations of another cell. That is to say, in such interference scenario, the interference which the uplink transmission of the femtocell mobile station causes to the uplink reception of the macro base station (in other words, uplink transmission of macro mobile station) is the subject. For this interference scenario, it is particularly problematic when the uplink frequency band of the femtocell and the macro cell are identical, the femtocell is the CSG cell, and the femtocell base station is arranged near the macro base station. In such case, since the transmission power of the macro mobile station which is arranged near the femtocell base station is relatively small, the interference caused by the uplink transmission of the femtocell mobile station to the uplink transmission of the macro mobile station is critical.
Non-Patent Literatures 1 and 2 and Patent Literature 1 disclose the technique to suppress the interference from the femtocell to the macro cell. Non-patent Literatures 1 and 2 each disclose controlling uplink transmission power of a femtocell by taking into account a propagation loss between the femtocell and the macro cell. To be more specific, Non-Patent Literature 1 discloses measuring a propagation loss between the femtocell base station and the macro base station, and performing the transmission power control so that the uplink transmission power of the femtocell mobile station becomes smaller as the propagation loss is smaller. The uplink transmission power control may be performed by adjusting uplink target received power at the femtocell base station or adjusting uplink maximum transmission power at the femtocell mobile station.
Patent Literature 1 focuses on that the number of femtocell base stations arranged within the macro cell changes, and aims to suppress the uplink interference to the macro cell from the plurality of femtocells arranged within the macro cell regardless of the number of the femtocell base stations within the macro cell. In order to achieve the aim, Patent Literature 1 discloses the following method. That is the method disclosed in Patent Literature 1 includes: calculating the number of femtocell base station arranged within the macro cell and with which a femtocell mobile station is communicating; dividing the permissible value of an entire uplink interference received at the macro base station by the calculated number of femtocell base station in order to estimate the upper limit of the uplink interference quantity permitted per femtocell base station; and setting the maximum transmission power of the femtocell mobile station by using the estimated upper limit of the uplink interference quantity and the propagation loss between the femtocell mobile station and the macro base station.